Demystifying Hazmat Chemistry

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STUDENT HANDOUT DEMYSTIFYING HAZMAT CHEMISTRY Although vapor density is independent of other factors, external conditions can have an effect on where a vapor or gas is found. Wind and air currents affect how well gases and vapors mix with the atmosphere and how easily they disperse. Moisture in the atmosphere can be absorbed by gases and vapors, causing them to be less buoyant. Temperature affects vapor pressure, which will cause gases and vapors to disperse at different rates. Likewise, atmospheric pressure at different elevations will either hold gases and vapors down or allow them to disperse more readily. Even concentration plays a part; lower concentrations of heavy gases and vapors won’t hug the ground as much as higher concentrations do. For example, although gasoline vapors are almost four times heavier than air, if the concentration of vapors is low (close to 1%), the vapors will mix more evenly with air. As Concentration Approaches 1%, Vapors Will Mix More Evenly with Air Continuing Challenge 2007 Page © 2007 Firebelle Productions and CHES
STUDENT HANDOUT DEMYSTIFYING HAZMAT CHEMISTRY Comparing Three Primary Threats We generally evaluate hazmat incidents in terms of three threats: oxygen deficiency, toxicity, and flammability. These three threats impact some of the decisions we have to make, for example, what personal protective equipment to wear or what community protective actions to implement. Toxicity Oxygen Deficiency Flammability Oxygen Deficiency The normal atmosphere is comprised of roughly 78% nitrogen, 20.9% oxygen, and 1.1% argon and other gases. Although 20.9% oxygen is considered normal, we don’t hit the “threat zones” until we drop below 19.5% or rise above 23.5%. The Limits of an Acceptable Oxygen Concentration If the oxygen level is above 23.5%, the atmosphere is considered to be potentially explosive. An oxygen-enriched atmosphere will cause any flammable or combustible material to ignite more readily and burn more fiercely. For the purposes of this discussion, however, we want to focus on oxygen-deficient atmospheres. An oxygen concentration of 19.5% is considered by OSHA and NIOSH to be the minimum safe level. Below that, responders must wear self-contained breathing apparatus (SCBA) or supplied air respirators (SAR). Air-purifying respirators (APR) are not acceptable under these conditions, even if they are capable of filtering contaminants present in the atmosphere, because they do not provide responders with a source of oxygen. Continuing Challenge 2007 Page © 2007 Firebelle Productions and CHES
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